CONCLUDING REMARKS. UO 
vertical horizon as, the place of their former occurrence. On the other hand, some of 
those previously considered rare have become more plentiful. 
In my list will be found many species to which my name is attached. Some of these 
will hereafter probably require to be altered through having been described by others 
previously to my work, but I have relinquished my own names wherever, up to the present 
time, I felt satisfied of the identity of my shell with any other previously described. In 
every case where I thought it could be justly identified with any other, whether known as 
living or not, I have not hesitated so to give it, suppressing, whenever occasion required, the 
name under which it had been previously known from the Crag in favour of any earlier 
name which the species with which it is thus identified may possess. After all this, 
however, there remains a wide difference between the view taken by me of the Crag 
Mollusca, and that taken by the author of the ‘ British Conchology ;’ indeed, while according 
to that author my tribute of admiration for the persevering industry with which he has 
so much enlarged eur knowledge of the Mollusca of British seas, and of the waters 
adjoining them, I do not hesitate to point out what, im my judgment, impairs the 
conclusions he has expressed with reference to Crag species, both in his general work, and 
in those lists of which he is the author which form part of Mr. Prestwich’s papers on 
‘the Crag already referred to. It is obvious that this author’s leanings are very marked, 
so as to group together allied Crag forms as varieties only of one species, and especially 
to make out a Crag shell to be either identical with a living species, or, at most, 
‘only a variety of it wherever the slightest presumption can be found for that course. I 
observe, however, that this reluctance to recognise two distinct, but allied, Crag forms as 
anything more than a variety gives way when the form has been found living; and I have 
been particularly struck with this in the case of Scalaria subulata, which in the first, or 
‘Cor. Crag List,’ is put in italics as a variety only of Scalaria foliacea, but which in a 
note to the ‘ Red Crag List’ (page 496), is restored to specific importance in consequence 
of its having in the meantime been dredged living by Mr. McAndrew. The degree of 
difference which is to constitute a species must in a great measure be arbitrary; but there 
are many shells of the Crag thus identified with living ones which show quite as little 
variation from shells of the Older and Middle 'Tertiaries as they do from their living 
analogues. If species arise, as I believe they do, by gradual variation from forms 
previously existing’ (be the cause of that variation what it may), it is obvious that, if we 
could get a perfect collection of all the forms that have existed and do exist on the surface 
of the earth, we should be placed in the dilemma of not being able to draw a specific line 
anywhere ; and although it must be a long day, if ever, before such a knowledge of animal 
" In a letter to me (April 27th, 1873) the author of the ‘ British Conchology’ says, “‘I believe not in 
evolution, but in descent with modification ;” and I observe the same phrase—“ descent with modification ” 
used by his colleague in the “Porcupine”? dredging expedition, Professor Thompson, in his late 
work, ‘The Depths of the Sea’ (page 480). I confess that I do not understand the difference between 
this and evolution, in which I haye for very many years been a believer. 
